Bearing monitoring technology using vibration signals obtained by accelerometers is well known and mature technology. However, this technology is not available for some important fields of application.
One important example is subsea devices such as deep sea pumps. Commonly used condition monitoring systems are difficult to deploy in subsea conditions due to the electronic packaging requirements and the high data bandwidth. Besides that, electronic transmission over long distances, for example when running alongside power lines, is sensitive to electromagnetic interference.
Deep sea pumps used for oil and gas applications are important assets for the production of an oil rig. Monitoring of the asset is problematic as the electronics can be less reliable than the mechanical system being monitored by the electronics. Therefore, for the purpose of bearing monitoring, conventional acceleration measurement is prohibited in these applications. Fibre optic sensors however can be applied as they are less complex. Fibre optic accelerometers that have been investigated are however not capable of delivering the required resolution for monitoring the condition of the bearing.
The document U.S. Pat. No. 4,196,629 discloses a system for monitoring the operation of ball bearings in rotating machinery by continuously measuring the radial deflections of the outer race of these bearings using a fibre optic proximity probe.
Fibre optic strain sensors do have sufficient resolution and frequency bandwidth to assess the condition. The interpretation of the signals however is not as straight forward as using frequency analysis for bearing defects detection by acceleration measurement or by proximity probes, as the measurement of strain is very localised.
A method using Fiber Bragg Gratings (FBG) is disclosed in the article “Load Sensing and Condition Monitoring for a Subsea-multiphase pump” by P. J. Feenstra and N. S. W. den Haak in the Proceedings of the 1st DSPE conference on precision mechatronics—Sep. 4-5, 2012. The authors propose using enveloped strain signals for the purpose of defect detection. However, it has turned out the reliability of this bearing defects detection can still be improved.